Higher performance, lower cost, increased miniaturization of components, and greater packing density of integrated circuits are ongoing goals of the computer industry. Greater integrated circuit density is primarily limited by the space or “real estate” available for mounting microelectronic component on a substrate such as a printed circuit board. The microelectronic component may be electrically connected to circuitry on the circuit board via flip chip attachments, wire bonding, tape automated bonding (TAB), or a variety of other techniques.
Increasingly, microelectronic components are being vertically stacked atop one another to conserve valuable substrate real estate. In such a vertically stacked assembly, a first microelectronic component is attached directly to the substrate and a second microelectronic component may be physically attached to the first microelectronic component (e.g., stacked on the first microelectronic component). If the first microelectronic component is electrically connected to the substrate via flip chip attachments or TAB, the active surface of the microelectronic component. (i.e., the surface bearing the electrical contacts for connection to the circuitry of the microelectronic component) faces toward the substrate. Commonly, the bare backside surface of the first microelectronic component is exposed and faces away from the substrate, and the second microelectronic component is attached directly to the backside surface.
If the first microelectronic component is electrically connected to the substrate by wire bonding, however, attachment of the second microelectronic component to the first microelectronic component can be more problematic. In wire-bonding techniques, the backside of the first microelectronic component is mounted to the substrate and the active surface of a wire bonded microelectronic component defines the outer surface which faces away from the substrate. The contacts on the active surface are then electrically coupled to the contacts on the substrate by small conductive “bonding” wires that extend from the active surface to the substrate. The wires that electrically connect the active surface of the microelectronic component to the substrate accordingly interfere with attaching the second microelectronic component directly on the active surface.
FIG. 1 schematically illustrates one current stacked microelectronic device in which the first and second microelectronic components are wire-bonded to the substrate. The stacked microelectronic device 100 of FIG. 1 includes a substrate 120 carrying a pair of microelectronic components 130 and 140. The substrate 120, which may be a circuit board or the like, has a contact surface 124 bearing a plurality of electrical contacts 126a–126d. A first microelectronic component 130 is attached to the component surface 124 of the substrate 120 by means of an adhesive 135. The adhesive 135 may cover the entire mounting face 132 of the first microelectronic component 130. The active surface 134 of the first microelectronic component 130 includes a plurality of electrical contacts 136a–136b. A first bonding wire 138a electrically connects the first electrical contact 136a of the first microelectronic component 130 to the first electrical contact 126a of the substrate 120, and a second bonding wire 138b electrically connects a second electrical contact 136b of the first microelectronic component 130 to a second electrical contact 126b of the substrate 120.
The second microelectronic component 140 is carried by the first microelectronic component 130. In some conventional stacked microelectronic devices, a facing surface 142 of the second microelectronic component is attached to the active surface 134 of the first microelectronic component 130 via an adhesive layer 145. This adhesive layer 145 conventionally has a thickness which is greater than the height to which the bonding wires 138 extend above the active surface 134 so the second microelectronic component 140 does not directly contact or rest against the bonding wires 138. Such a structure is shown in U.S. Pat. No. 5,323,060 (Fogal et al.), the entirety of which is incorporated herein by reference. Once the second microelectronic component 140 is in place, a first electrical contact 146a on the outer surface 144 of the second microelectronic component 140 can be electrically connected to a third electrical contact 126c carried by the substrate 120. Similarly, a second electrical contact 146b on the outer surface 144 can be electrically connected to a fourth electrical contact 126d carried by the substrate 20.
The stacked microelectronic device of FIG. 1 includes two microelectronic components 130 and 140. The same approach can be employed to stack three or more microelectronic components in a single microelectronic device. For example, a third microelectronic component (not shown) may be attached to the outer surface 144 of the second microelectronic component 140 using another adhesive layer similar to adhesive layer 145. The third microelectronic component can be joined to other electrical contacts carried by the substrate 120 (e.g., contacts 126e and 126f) or one of the other microelectronic components (e.g., contacts 146) via wire bonding.
The system proposed by Fogal et al. provides a relatively simple structure which enables stacking of microelectronic components to increase component density in a microelectronic device. However, a stacked microelectronic device such as that shown in FIG. 1 can present some manufacturing difficulties. For example, rapidly and precisely positioning the adhesive layer 145 and the second microelectronic component 140 can be a challenge. For this structure to work reliably, the adhesive layer 145 must be positioned within a central area of the first component's active surface 134 inside of the electrical contacts 136 of the first microelectronic component. If the adhesive layer 145 overlaps the electrical contacts 136, this can damage or interfere with the connection between the electrical contacts 136 and the associated bonding wires 138. Many microelectronic components, such as semiconductor dies, are fairly small. It can be difficult to consistently and unerringly position the adhesive layers 145 on previously mounted microelectronic components 130 in a rapid fashion to facilitate mass production of stacked microelectronic devices 100. After the adhesive layer 145 is applied, the next microelectronic component 140 must be accurately positioned on the adhesive layer 145. As the number of microelectronic components stacked atop one another in the microelectronic device increases, the chances for error increase concomitantly as one error in alignment or position of any layer of the stacked device can render the entire device unacceptable.